The present invention relates to thermal oxidation of waste, and more particularly to a controlled process for two stage thermal oxidation of selected solid wastes to significantly reduce targeted air emissions.
The process of two stage combustion is an old art in which combustible materials are normally burned under substoichiometric conditions in the first stage chamber to produce combustible gases and ash. The resultant combustible gases are further mixed with air and burned under superstoichiometric conditions in the second stage.
The control of two stage combustion is typified in U.S. Pat. Nos. 4,013,203 and 4,182,246 wherein reverse action air control and auxiliary fuel fired burners are used to control first stage operating temperatures within a specified range while concurrently assuring substoichiometric conditions by further over-riding air and auxiliary burner requirements, when necessary, to maintain a certain oxygen content in the combustible gases passing into the secondary stage. The second stage temperature is controlled by direct mode since an increase in secondary temperature results in an increase in air flow causing quenching effects on combusting gases and lower temperature. A further complication is encountered in temperature control when air flow requirements are over-ridden and increased whenever a certain minimum level of oxygen is not maintained in the secondary exit gasses.
Improvements for the control of typified two stage combustion systems are documented in U.S. Pat. No., 4,474,121 which concentrates on assuring substoichiometric conditions in the first stage and controlled superstoichiometric air rates in the second stage which in essence eliminates any requirement for oxygen monitoring of first stage exit gases and provides for substantially better control of the combustion process compared to earlier technologies.
Other patents of general background interest, describing and illustrating waste incineration methods and apparatus, include:
______________________________________ U.S. Pat. No. 3,595,181 Anderson et al. July 27, 1971 U.S. Pat. No. 3,610,179 Shaw, Jr. et al. October 5, 1971 U.S. Pat. No. 3,651,771 Eberle et al. March 28, 1972 U.S. Pat. No. 3,664,277 Chatterjee et al. May 23, 1972 U.S. Pat. No. 3,680,500 Pryor et al. August 1, 1972 U.S. Pat. No. 4,517,906 Lewis et al. May 21, 1985 U.S. Pat. No. 4,800,824 DiFonzo et al. January 31, 1989 U.S. Pat. No. 4,870,910 Wright et al. October 3, 1989 U.S. Pat. No. 4,941,415 Pope et al. July 17, 1990 U.S. Pat. No. 4,976,207 Richard et al. December 11, 1990 U.S. Pat. No. 5,095,829 Nevels et al. March 17, 1992 U.S. Pat. No. 5,123,364 Gitman et al. June 23, 1992 U.S. Pat. No. 5,222,446 Edwards et al. June 29, 1993 ______________________________________
These typified control systems do not address the air emission problems associated with highly variable air flow rates passing through the combusting materials within the first stage which can cause dramatic increases in ash particulate entrainment and necessitate the use of particulate removal systems before exhaust gases can exit into heat exchangers or the atmosphere. The constant fouling of analytical instruments used to monitor the composition of first stage exist gases results in inaccurate readings and necessitates constant vigilance and maintenance to provide the desired process control.
Accordingly it is an object of the present invention to provide a combustion oxidation process which is adapted to meet specific, internationally acceptable air quality assurances without the necessity of costly exhaust gas scrubbing and filtration to remove organic compounds and solid particulates.